An optical fiber distribution cable generally comprises two or more optical fibers enclosed within a jacket. Cables designed for long horizontal runs in overhead ladder racks and underfloor trays in service provider central office and data center facilities, commonly referred to as horizontal backbone cables, have high tensile strength to withstand the pull tension of the installation process and high stiffness to promote crush and kink resistance and protect the relatively fragile glass optical fibers. To provide tensile strength, Kevlar® or other aramid yarn may be included in the cable in the form of a reinforcing layer between the fibers and the jacket. To provide crush and kink resistance, an acrylate matrix layer may be provided between the fibers and the aramid yarn layer. An example of such a cable is the OFS AccuPack® cable, available with 24 fibers in a 5.4 mm diameter and with 12 fibers in a 4.5 mm diameter.
As bandwidth demands continue to increase, service providers and data center operators often must serve ever more data from existing physical facilities, such as central offices and edge data centers. These facilities have limited capacity for cabling in existing overhead ladder racks and underfloor trays. To accommodate increasing bandwidth demands with existing rack and tray capacity, the fiber packing density of cables used in these environments may be increased. The outside diameter of the cable is a limiting factor on the fiber packing density, i.e., number of optical fibers that can be included. Increasing the packing density without increasing the cable diameter leaves less space for aramid reinforcing yarn.
Flaws on the surface of glass optical fibers can cause the fiber to break at a relatively low level of tensile strain. As described in International Electrotechnical Commission (IEC) Technical Recommendation 62048, the larger the flaw, the lower the strain required to cause the fiber to break. In order to screen out such flaws, conventional optical fibers with a glass cladding diameter of 125 microns are typically subjected to a proof test during the manufacturing process. As described in standards such as IEC 60793-2-59 and Telcordia GR-20, the standard proof test force for conventional optical fiber with 125 micron diameter glass is 0.69 gigapascal (GPa), resulting in a proof strain of 1.0 percent.
Optical fibers must be packaged in cables that provide mechanical protection against crushing and tensile loads. Common North American industry standards, such as Telcordia GR-409 or ICEA-S-596, require that cables be designed so that the maximum strain on the fiber during installation be less than 0.6 percent of the fiber proof strain. This limit provides a factor of safety during field installation. For the case of conventional fiber with a glass diameter of 125 microns proof tested at 0.69 GPa load, this means a fiber strain limit of 0.6 percent at standard installation tensile load.
North American industry standard installation tensile load requirements for optical fiber backbone cables are 440 Newtons (N) for a fiber in a cable having twelve or fewer fibers, and 660 N for a fiber in a cable having more than twelve fibers. Providing a compact, i.e., small diameter, optical fiber cable that meets these requirements presents challenges, which may be addressed by the present invention in the manner described below.